Re: Negative Welch-Stetson Index

[Andrew Bennett <andrew.bennett@ns.sympatico.ca>]

On Sun, 09 Jun 2002 19:07:42 +0000, jg <jg@jgws.freeisp.co.uk>
wrote:

> ... I came across object 34449 from collected.big
>
>It had a negative Welch-Stetson index.

>It seems that negative Welch-Stetson indices flag problem multi-epoch
>two colour photometric data that can emulate changes due to
>variability.  If I've read it right.

That's a mouthful!

If all the stars were really constant and the
observations were perfect apart from random noise,
half the W-S values would be negative.
 
>1869 stars of the 94,500+ stars in collected.big from CD 23 gave a
>negative WS index, and of those a mere 180 are less than WS index = -1

Only 2% negative. Um! If 1% of stars were variable,
you would expect 49.5 % negative W-S values.

Welcome to the real world of systematic errors!

As you can see, the overwhelming majority of systematic
errors affect both cameras the same way: roofs, oak
trees, clouds, you name it.

And 0.2% affect the two cameras opposite ways, giving
W-S < -1. Any guesses as to how? My first question - are
these stars close to the edge of some of the images?

Andrew Bennett, Avondale Vineyard